Method of repairing an acrylic denture base and zirconia autopolymerizable resins therof

ABSTRACT

A method of repairing an acrylic denture base employing an autopolymerizable acrylic reinforcement resin comprising zirconium dioxide nanoparticles is disclosed. Additionally, a kit comprising precursors of the acrylic reinforcement resin, the acrylic reinforcement resin, and an acrylic denture base comprising the acrylic reinforcement resin are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. Provisional Application No. 62/503,706 filed May 9, 2017, the entire contents of which are herein incorporated by reference.

BACKGROUND OF THE DISCLOSURE Technical Field

The present disclosure relates to a method of repairing an acrylic denture base employing an autopolymerizable acrylic reinforcement resin comprising zirconium dioxide nanoparticles, as well as a kit comprising precursors of the acrylic reinforcement resin, the acrylic reinforcement resin, and an acrylic denture base comprising the cured acrylic reinforcement resin.

Description of the Related Art

The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present disclosure.

Denture fracture is a common problem in prosthodontic practice that troubles both patients and prosthodontists. Inordinate masticatory forces or denture deformation during use can result in bending forces that contribute to fatigue of the material and subsequent fracture [U. R. Darbar, R. Huggett, and A. Harrison, “Denture fracture: a survey,” British Dental Journal, vol. 176, no. 9, pp. 342-345, 1994.—incorporated herein by reference in its entirety]. A new denture construction is costly and time consuming so denture repair is preferred [R. S. Se'o, K. H. Neppelenbroek, and J. N. A. Filho, “Factors affecting the strength of denture repairs: topics of interest,” Journal of Prosthodontics, vol. 16, no. 4, pp. 302-310, 2007.—incorporated herein by reference in its entirety]. Satisfactory repair should be easy and rapid and match the original color of the denture base while maintaining the dimensional accuracy [G. L. Polyzois, P. A. Tarantili, M. J. Frangou, and A. G. Andreopoulos, “Fracture force, deflection at fracture, and toughness of repaired denture resin subjected to microwave polymerization or reinforced with wire or glass fiber,” Journal of Prosthetic Dentistry, vol. 86, no. 6, pp. 613-619, 2001.—incorporated herein by reference in its entirety]. Denture repair depends on many variables including material type, material reinforcement, surface design and surface treatment. Several materials have been used to repair fractured denture bases, including autopolymerized, visible light polymerized, heat polymerized, or microwave polymerized acrylic resin [J. N. Arioli Filho, L. E. Butignon, R. D. P. Pereira, M. G. Lucas, and F. D. A. Mollo Jr., “Flexural strength of acrylic resin repairs processed by different methods: water bath, microwave energy and chemical polymerization,” Journal of Applied Oral Science, vol. 19, no. 3, pp. 249-253, 2011; and S. Suvarna, T. Chhabra, D. Raghav, D. Singh, P. Kumar, and S. Sahoo, “Residual monomer content of repair autopolymerizing resin after microwave post polymerization treatment,” European Journal of Prosthodontics, vol. 2, no. 1, pp. 28-32, 2014.—each incorporated herein by reference in its entirety]. Most (˜86%) of denture base repairs are made with autopolymerized acrylic resin because it is easy to manipulate, fast, and can be used chair-side [A. I. Zissis, G. L. Polyzois, and S. A. Yannikakis, “Repairs in complete dentures: results of a survey,” Quintessence of Dental Technology, vol. 20, pp. 149-155, 1997; and C. Bural, G. Bayraktar, I. Aydin, I. Yusufo ̆glu, N. Uyumaz, and M. Hanzade, “Flexural properties of repaired heat-polymerizing acrylic resin after wetting with monomer and acetone,” Gerodontology, vol. 27, no. 3, pp. 217-223, 2010.—each incorporated herein by reference in its entirety]. Unfortunately, its strength has been shown to range from 18 to 81% of intact heat polymerized denture resins [I. Kostoulas, V. T. Kavoura, M. J. Frangou, and G. L. Polyzois, “Fracture force, deflection, and toughness of acrylic denture repairs involving glass fiber reinforcement,” Journal of Prosthodontics, vol. 17, no. 4, pp. 257-261, 2008.—incorporated herein by reference in its entirety], and in general heat-polymerized materials have been proven to have better mechanical properties than autopolymerized resin [Alkurt M, Yeşil Duymuş Z, Gundogdu M. Effect of repair resin type and surface treatment on the repair strength of heat-polymerized denture base resin. J Prosthet Dent. 2014; 111(1):71-78; and Faot F, da Silva W J, da Rosa R S, et al. Strength of denture base resins repaired with auto- and visible light polymerized materials. J Prosthodont. 2009; 18(6):496-502; and Beyli M S, von Fraunhofer J A. Repair of fractured acrylic resin. J Prosthet Dent. 1980; 44(5):497-503; and Agarwal M, Nayak A, Hallikerimath R B. A study to evaluate the transverse strength of repaired acrylic denture resins with conventional heat-cured, autopolymerizing and microwave-cured resins: an in vitro study. J Ind Prosthodont Soc. 2008; 8:36-41; and Anusavice K J. Philips Science of Dental Materials. Bangalore, India: Prism Saunders, Prism book Pvt Ltd; 2002:124-136.—each incorporated herein by reference in its entirety].

Many attempts have been made to overcome this shortcoming via using reinforced repair material and/or modification of repair surface design and treatment. Hanna, et al. investigated the effect of a 45° bevel of the repair surface on the transverse strength of the repaired denture base and found that higher values were obtained [E. A. Hanna, F. K. Shah, and A. A. Gebreel, “Effect of joint surface contours on the transverse and impact strength of denture base resin repaired by various methods: an in vitro study,” Journal of American Science, vol. 6, no. 9, pp. 115-125, 2010.—incorporated herein by reference in its entirety]. Beveling of the repair surface changed the fracture type from a weak adhesive to a strong cohesive fracture [S. Kirti, M. R. Dhakshaini, and A. K. Gujjari, “Evaluation of transverse bond strength of heat cured acrylic denture base resin repaired using heat polymerizing, autopolymerizing and fiber reinforced composite resin—an in vitro study,” International Journal of Clinical Cases and Investigations, vol. 4, no. 2, pp. 33-43, 2012.—incorporated herein by reference in its entirety]. It is appropriate to treat the repair surface with repair monomer as it modifies the surface structure and increases its bond to repair material [P. K. Vallittu, V. P. Lassila, and R. Lappalainen, “Wetting the repair surface with methyl methacrylate affects the transverse strength of repaired heat-polymerized resin,” The Journal of Prosthetic Dentistry, vol. 72, no. 6, pp. 639-643, 1994; and M. Vojdani, S. Rezaei, and L. Zareeian, “Effect of chemical surface treatments and repair material on transverse strength of repaired acrylic denture resin,” Indian Journal of Dental Research, vol. 19, no. 1, pp. 2-5, 2008; and H. Minami, S. Suzuki, Y. Minesaki, H. Kurashige, and T. Tanaka, “In vitro evaluation of the influence of repairing condition of denture base resin on the bonding of autopolymerizing resins,” Journal of Prosthetic Dentistry, vol. 91, no. 2, pp. 164-170, 2004.—each incorporated herein by reference in its entirety].

Glass fiber is one of the most common reinforcement materials and many investigations of its effect on repaired denture bases have been performed. Addition of glass fiber to repair material improves the strength of a denture base repair and may decrease the occurrence of further fracture [H. D. Stipho, “Repair of acrylic resin denture base reinforced with glass fiber,” The Journal of Prosthetic Dentistry, vol. 80, no. 5, pp. 546-550, 1998; and E. Nagai, K. Otani, Y. Satoh, and S. Suzuki, “Repair of denture base resin using woven metal and glass fiber: effect of methylene chloride pretreatment,” The Journal of Prosthetic Dentistry, vol. 85, no. 5, pp. 496-500, 2001.—each incorporated herein by reference in its entirety]. This may be attributed to the fact that glass fiber has a high resilience which allows the stresses to be received by them without deformation [G. Uzun, N. Hersek, and T. Tincer, “Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin,” The Journal of Prosthetic Dentistry, vol. 81, no. 5, pp. 616-620, 1999.—incorporated herein by reference in its entirety].

Zirconia (ZrO₂) is a metal oxide and has been used as a reinforcement material to improve the transverse strength of denture base resin (i.e. heat-polymerized resins) [N. V. Asar, H. Albayrak, T. Korkmaz, and I. Turkyilmaz, “Influence of various metal oxides on mechanical and physical properties of heat-cured polymethylmethacrylate denture base resins,” Journal of Advanced Prosthodontics, vol. 5, no. 3, pp. 241-247, 2013; and A. O. Alhareb and Z. A. Ahmad, “Effect of Al₂O₃/ZrO₂ reinforcement on the mechanical properties of PMMA denture base,” Journal of Reinforced Plastics and Composites, vol. 30, no. 1, pp. 86-93, 2011.—each incorporated herein by reference in its entirety]. It possesses high mechanical strength, good surface properties, and good biocompatibility and biological properties, thus making it a beneficial material for use in dental materials, such as reinforcement of denture bases and repair [Ayad N M, Badawi M F, Fatah A A. Effect of reinforcement of high impact acrylic resin with micro-zirconia on some physical and mechanical properties. Rev Clin Pesq Odontol Curitiba. 2008; 4(3):145-151; and Asar N V, Albayrak H, Korkmaz T, Turkyilmaz I. Influence of various metal oxides on mechanical and physical properties of heat-cured polymethylmethacrylate denture base resins. J Adv Prosthodont. 2013; 5(3):241-247; and Shuai C, Feng P, Yang B, Cao Y, Min A, Peng S. Effect of nano-zirconia on the mechanical and biological properties of calcium silicate scaffolds. Int J Appl Ceramic Technol. 2015; 12:1148-1156; and Mallineni S K, Nuvvula S, Matinlinna J P, Yiu C K, King N M. Biocompatibility of various dental materials in contemporary dentistry: a narrative insight. J Invest Clin Dent. 2013; 4(1):9-19.—each incorporated herein by reference in its entirety]. Reinforcement of acrylic denture base with zirconia significantly increases its transverse strength [N. M. Ayad, M. F. Badawi, and A. A. Fatah, “The effect of reinforcement of high-impact acrylic resin with zirconia on some physical and mechanical properties,” Cairo Dental Journal, vol. 24, no. 2, pp. 245-250, 2008.—incorporated herein by reference in its entirety]. Recently, nanotechnology has been used in the prosthodontic field for medical and material enhancement purposes. The properties of the reinforced resin modified by nanoparticles depend on the size, shape, type and concentration of the added particles [I. N. Safi, “Evaluation the effect of nano-fillers (TiO₂, Al₂O₃, SiO₂) addition on glass transition temperature, E-modulus and coefficient of thermal expansion of acrylic denture base material,” Journal of Baghdad College of Dentistry, vol. 26, no. 1, pp. 37-41, 2014.—incorporated herein by reference in its entirety]. Additions of nano-zirconia to polymethyl methacrylate (PMMA) have been reported to increase the transverse strength of base resins (i.e. heat polymerized resins) due to its small size and homogenous distribution [N. S. Ihab and M. Moudhaffar, “Evaluation the effect of modified nano-fillers addition on some properties of heat cured acrylic denture base material,” Journal of Baghdad College of Dentistry, vol. 23, no. 3, pp. 23-29, 2011.—incorporated herein by reference in its entirety].

Polymethyl methacrylate (PMMA) is the most common denture base material due to its biocompatibility, esthetics, accurate fit, stability in the oral environment, ease of fabrication and adjustment, low cost, and possibility of repair [Meng T R, Latta M A. Physical properties of four acrylic denture base resins. J Contemp Dent Pract. 2005; 6(4):93-100; and Nejatian T, Johnson A, Van Noort R. Reinforcement of denture base resin. Adv Sci Technol. 2006; 49:124-129.—each incorporated herein by reference in its entirety]. However, PMMA has poor mechanical properties, which often results in denture base fractures [Raszewski Z, Nowakowska D. Mechanical properties of hot curing acrylic resin after reinforced with different kinds of fibers. Int J Biomed Mat Res. 2013; 1:9-13.—incorporated herein by reference in its entirety]. Such denture fractures may occur inside the patient's mouth usually at the midline of the denture base during mastication or outside the patient's mouth when the removable prosthesis drops suddenly [Beyli M S, von Fraunhofer J A. An analysis of causes of fracture of acrylic resin dentures. J Prosthet Dent. 1981; 46(3):238-241; and Darbar U R, Huggett R, Harrison A. Denture fracture—a survey. Br Dent J. 1994; 176(9):342-345; and Polyzois G L, Andreopoulos A G, Lagouvardos P E. Acrylic resin denture repair with adhesive resin and metal wires: effects on strength parameters. J Prosthet Dent. 1996; 75(4):381-387.—each incorporated herein by reference in its entirety].

The effects of nano-ZrO₂ on the repair strength of PMMA denture bases with autopolymerized resins have not been investigated in the literature, however the literature expectation is that nano-ZrO₂, even at varying concentrations, will not improve the flexural strength and impact strength of repaired PMMA denture bases.

In view of the forgoing, one object of the present disclosure is to provide a method of repairing an acrylic denture base employing an autopolymerizable acrylic reinforcement resin comprising zirconium dioxide nanoparticles, and a kit for making such repairs, that produces repaired denture bases having advantageous flexural strength and impact strength properties.

BRIEF SUMMARY OF THE DISCLOSURE

According to a first aspect, the present disclosure relates to a method of repairing an acrylic denture base having a repair gap surface, the method comprising i) painting the repair gap surface of the acrylic denture base with a monomer liquid comprising methyl methacrylate thereby forming a painted repair gap surface, ii) dispersing a nanocomposite powder in a second amount of the monomer liquid comprising methyl methacrylate thereby forming a reinforcement resin, the nanocomposite powder comprising a) an acrylic polymer powder comprising polymethyl methacrylate and b) zirconium dioxide nanoparticles, iii) applying and packing the reinforcement resin in excess to the painted repair gap surface of the acrylic denture base, and iv) autopolymerizing the reinforcement resin thereby forming a repaired acrylic denture base.

In one embodiment, the nanocomposite powder comprises 1-10 wt % zirconium dioxide nanoparticles relative to the total weight of the nanocomposite powder.

In one embodiment, the zirconium dioxide nanoparticles have an average granularity of 50-130 nm.

In one embodiment, the zirconium dioxide nanoparticles have an average surface area of 5-15 m²/g.

In one embodiment, the reinforcement resin has a nanocomposite powder to monomer liquid mass ratio in a range of 0.5:1 to 3:1.

In one embodiment, the autopolymerizing is performed at a temperature of 20-50° C.

In one embodiment, the autopolymerizing is performed at a temperature of 20-30° C.

In one embodiment, the autopolymerizing is performed at a pressure of 10-50 psi.

In one embodiment, the method further comprises treating the zirconium dioxide nanoparticles with a silane coupling reagent prior to the dispersing.

In one embodiment, the treating comprises immersing the zirconium dioxide nanoparticles in a solution comprising 1.0-5.0 g of the silane coupling reagent per liter of the solution.

In one embodiment, the silane coupling reagent is 3-(trimethoxysilyl)propyl methacrylate.

In one embodiment, the repaired acrylic denture base has a flexural or transverse strength of 60-95 MPa.

In one embodiment, the repaired acrylic denture base has an impact strength of 1.5-2.5 kJ/m².

In one embodiment, the repair gap surface is in a form of at least one selected from the group consisting of a bevel joint, a butt joint, a rabbet surface, and a round surface.

In one embodiment, the repair gap surface is in a form of a bevel joint.

According to a second aspect, the present disclosure relates to a kit comprising i) a monomer liquid comprising methyl methacrylate and ii) a nanocomposite powder comprising a) an acrylic polymer powder comprising polymethyl methacrylate and b) 1-10 wt % zirconium dioxide nanoparticles relative to the total weight of the nanocomposite powder and an average granularity of 50-130 nm and an average surface area of 5-15 m²/g, wherein the monomer liquid and the nanocomposite powder are suitable for forming a reinforcement resin that autopolymerizes at a temperature of 20-40° C. and a pressure of 10-30 psi.

In one embodiment, the kit further comprises at least one selected from the group consisting of a silane coupling reagent, a polymerization accelerator, a cross-linking agent, and an adhesion promoter.

In one embodiment, the kit further comprises a silane coupling reagent which is 3-(trimethoxysilyl)propyl methacrylate.

According to a third aspect, the present disclosure relates to an acrylic reinforcement resin comprising i) polymethyl methacrylate formed by an autopolymerization reaction of methylmethacrylate at a temperature of 20-40° C., and ii) zirconium dioxide nanoparticles having an average granularity of 50-130 nm and an average surface area of 5-15 m²/g.

According to a fourth aspect, the present disclosure relates to an acrylic denture base comprising the acrylic reinforcement resin in any of its embodiments having a flexural or transverse strength of 60-95 MPa and an impact strength of 1.5-2.5 kJ/m².

The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the mean value of transverse strength in MPa for the intact heat polymerized specimen (HC, control), the autopolymerized acrylic resin repair specimen (AP), the autopolymerized acrylic resin reinforced with 2 wt % glass fiber repair specimen (2GF), the autopolymerized acrylic resin reinforced with 5 wt % glass fiber repair specimen (5GF), the autopolymerized acrylic resin reinforced with 2 wt % zirconia repair specimen (2ZR), the autopolymerized acrylic resin reinforced with 5 wt % zirconia repair specimen (5ZR), the autopolymerized acrylic resin reinforced with 2 wt % nano-zirconia repair specimen (2NZR), and the autopolymerized acrylic resin reinforced with 5 wt % nano-zirconia repair specimen (5NZR)

FIG. 2 is a schematic representation of the molds, intact specimens, and modified plates (butt joint and bevel joint) used for the fabrication of specimens and the repair of specimens (butt surface design and bevel surface design repair).

FIG. 3 is a scanning electron microscopy (SEM) image of a fractured surface of the intact heat-polymerized control flexural strength specimen.

FIG. 4 is a SEM image of a fractured surface of the bevel joint 2.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired flexural strength specimen.

FIG. 5 is a SEM image of a fractured surface of the bevel joint 5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired flexural strength specimen.

FIG. 6 is a SEM image of a fractured surface of the bevel joint 7.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired flexural strength specimen.

FIG. 7 is a SEM image of a fractured surface of the butt joint 2.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired flexural strength specimen.

FIG. 8 is a SEM image of a fractured surface of the butt joint 5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired flexural strength specimen.

FIG. 9 is a SEM image of a fractured surface of the bevel joint 2.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired impact strength specimen.

FIG. 10 is a SEM image of a fractured surface of the bevel joint 5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired impact strength specimen.

FIG. 11 is a SEM image of a fractured surface of the bevel joint 7.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired impact strength specimen.

FIG. 12 is a SEM image of a fractured surface of the butt joint 2.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired impact strength specimen.

FIG. 13 is a SEM image of a fractured surface of the butt joint 5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired impact strength specimen.

FIG. 14 is a SEM image of a fractured surface of the butt joint 7.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin repaired impact strength specimen.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all of the embodiments of the disclosure are shown.

As used herein, the words “a” and “an” and the like carry the meaning of “one or more”. Additionally, within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

According to a first aspect, the present disclosure relates to a method of repairing an acrylic denture base having a repair gap surface, the method comprising i) painting the repair gap surface of the acrylic denture base with a monomer liquid comprising methyl methacrylate thereby forming a painted repair gap surface, ii) dispersing a nanocomposite powder in a second amount of the monomer liquid comprising methyl methacrylate thereby forming a reinforcement resin, the nanocomposite powder comprising a) an acrylic polymer powder comprising polymethyl methacrylate and b) zirconium dioxide nanoparticles, iii) applying and packing the reinforcement resin in excess to the painted repair gap surface of the acrylic denture base, and iv) autopolymerizing the reinforcement resin thereby forming a repaired acrylic denture base.

As used herein, “painting” is defined as applying a liquid, such as a liquid useful for repairing a dental prosthesis, directly to a surface, notch, crack, or groove of the dental prosthesis. Such application can be accomplished by spraying, dropping liquid droplets from a dropper, application through contact with a brush, etc. In one embodiment, after the painting, only the monomer liquid is present in the repair gap i.e. none of the monomer liquid polymerizes at this stage or essentially none, meaning the monomer liquid still behaves essentially like a pure monomer liquid even if trace amounts of the monomer has underwent polymerization. Alternatively, the painting may also involve partial (incomplete) curing of the monomer liquid after the application via electron beam curing, heat induced curing, radical initiated curing, ultrasound induced curing, and/or UV curing, for example, to form a semi-liquid material within the repair gap.

Types of dental prostheses that can be repaired using the method disclosed herein are generally known to those of ordinary skill, and specifically includes a crown, a bridge, an implant, an orthodontic device, and a denture. The method disclosed herein can therefore be applied to a multitude of dental uses including denture repair and relining, interim prosthesis construction, orthodontic appliance construction/repair, interim fixed prostheses (e.g. temporary crown and bridges), maxillofacial prosthesis, interim prosthesis during implant procedures, as many others. Any amount of the monomer liquid may be applied to the repair gap surface of the acrylic denture base, and one of ordinary skill in the art can adjust the amount depending on the size of the repair gap or dental prosthesis needed to be repaired.

As used herein “acrylic resin” refers to a group of related thermoplastic or thermosetting plastic substances derived from acrylic acid, methacrylic acid or other related compounds. As used herein polymethyl methacrylate or poly(methyl methacrylate) or PMMA, also known as acrylic or acrylic glass as well as by the trade names Plexiglas, Acrylite, Lucite, and Perspex among several others refers to a typically transparent thermoplastic.

The monomer liquid comprises a polymerization compound having free radically active functional groups and includes monomers, oligomers, and polymers having one or more ethylenically unsaturated groups such as acryl, methacryl, vinyl and the like. Preferably, the monomer liquid comprises a monomer comprising a mono-functional ethylenically unsaturated monomer (i.e. a monomer containing one ethylenically unsaturated group) and/or a multifunctional ethylenically unsaturated monomer (i.e. a monomer containing more than one ethylenically unsaturated group). Di- or poly-functional monomers such as dimethacrylates, which are the major constituents of dental filling materials may constitute only a small portion of the monomer liquid, such as less than 5 wt %, less than 4 wt %, less than 3 wt %, less than 2 wt %, less than 1 wt % relative to the total weight of the monomer liquid.

Preferably, according to the invention, the mono-functional or multifunctional monomer is an acrylate or a methacrylate derivative, more preferably a methacrylate derivative such as, but not limited to, mono-, di- or polymethacrylate including the mono-functional methyl methacrylate (MMA), isopropyl methacrylate, isobornyl-methacrylate, phenoxyethylmethacrylate, tetrahydrofurfiurylmethacrylate, cyclohexyl methacrylate, isophoryl methacrylate; multifunctional methacrylates including triethyleneglycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, trimethylolpropane trimethacrylate, 1,2,4-butanetriol trimethacrylate, 1,6-hexanediol dimethacrylate, pentaerythritol tetramethacrylate; 2,2-bis [4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane (Bis-GMA); 2,2-bis[4-(methacry-4loyloxy-ethoxy)phenyl]propane (or ethoxylated bisphenol A-dimethacrylate) (EBPADMA); urethane dimethacrylate (UDMA), diurethane dimethacrylate (DUDMA); polyurethane dimethacrylate (PUDMA); polycarbonate dimethacrylate (PCDMA); the bis-methacrylates of polyethylene glycols; copolymerizable mixtures of mathacrylated monomers; methacrylated oligomers; bis[2-(methacryloxyloxy)ethyl] phosphate; and mixtures thereof.

Methacrylates are most useful since they form relatively hard polymers and since they are relatively non-irritant and non-toxic as compared to acrylic monomers for example. The simplest methacrylate, methyl methacrylate (MMA), forms a polymer with a high glass transition temperature (Tg) of about 100° C. (referred to herein as a “hard polymer”), and is one of the preferred monomer liquid components. The glass transition temperature is a function of chain flexibility. The glass transition occurs when there is enough vibrational (thermal) energy in the system to create sufficient free-volume to permit sequences of 6-10 main-chain carbons to move together as a unit. At this point, the mechanical behavior of the polymer changes from rigid and brittle to tough and leathery, a behavior defined herein as a “plastic behavior”.

Thus, preferred mono-functional methacrylate derivatives useful for producing the reinforcement resin of the present invention include, but are not limited to, methyl methacrylate MMA, isobornyl-methacrylate which forms a hard polymer with Tg of 110° C., phenoxyethylmethacrylate (Tg of 54° C.), tetrahydrofurfurylmethacrylate (Tg of 75° C.), cyclohexyl methacrylate and isophoryl methacrylate. The most preferred mono-functional methacrylate component of the monomer liquid is MMA. Other useful methacrylates include also more flexible methacrylate monomers such as ethyl, isopropyl, butyl, iso-butyl and t-butylmethacrylates; 2-ethylhexylmethacrylate, hexylmethacrylate, nonylmethacrylate, lauryl methacrylate and other linear and branched alkyl methacrylates; benzyl methacrylate, phenyl methacrylate, t-butylphenyl methacrylate, 2-methoxyethyl methacrylate, 2-ethoxyethyl methacrylate, and 2-butoxyethyl methacrylate. In a preferred embodiment, the monomer liquid contains at least 20 wt %, or at least 30 wt %, or at least 40 wt %, or at least 50 wt %, or at least 60 wt %, or at least 70 wt %, or at least 80 wt %, or at least 90 wt %, or at least 95 wt % methyl methacrylate monomer relative to the total weight of the monomer liquid.

Further preferred methacrylate derivatives suitable for the purpose of the invention are those formed by the reaction of methacrylic acid with a monofunctional glycidyl ethers (epoxies) such as butyl glycidyl ether, cresyl glycidyl ether, phenyl glycidyl ether, and other epoxies such as limonene oxide. Also preferred are carbamate (urethane) adducts of hydroxyethyl methacrylate or hydroxypropyl methacrylate with mono functional isocyanates including, but not limited to, methylisocyanate, ethylisocyanate, propylisocyanate, isopropylisocyanate, butylisocyanate, isobutylisocyanate, phenylisocyanate, or an adduct of isocyanatoethylmethacrylate with an hydroxy compound such as methanol, ethanol and the like.

The acrylic polymer used to form the nanocomposite powder may preferably contain polymethyl methacrylate, and even more preferably polymethyl methacrylate formed by an auto-polymerization process.

As used herein zirconium dioxide (ZrO₂), also known as zirconia, is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. Zirconia may be produced by calcining zirconium compounds, exploiting its high thermal stability. Three phases are known: monoclinic (˜<1170° C.), tetragonal (˜1170-2370° C.), and cubic (˜>2370° C.). The trend is for higher symmetry at higher temperatures, as is usually the case. In certain embodiments, a few percentage of the oxides of calcium or yttrium stabilize the cubic phase. The rare mineral tazheranite (Zr, Ti, Ca)O₂ is cubic. Unlike TiO₂, which features six-coordinate Ti in all phases, monoclinic zirconia consists of seven-coordinate zirconium centers. This difference may be attributed to the larger size of the Zr atom relative to the Ti atom.

ZrO₂ adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The change of volume caused by transitions induces large stresses, which may cause it to crack upon cooling from high temperatures. When zirconia is blended with other oxides, the tetragonal and/or cubic phases may be stabilized. Exemplary effective dopants include, but are not limited to, magnesium oxide (MgO), yttrium oxide (Y₂O₃, yttria), calcium oxide (CaO), and cerium (III) oxide (Ce₂O₃).

In a preferred embodiment, the nanocomposite powder comprises 1-10 wt % zirconium dioxide nanoparticles relative to the total weight of the nanocomposite powder, preferably 1.5-8 wt %, preferably 2-7.5 wt %, preferably 2.5-7.0 wt %, preferably 2.5-6 wt %, preferably 3-5 wt % zirconium dioxide nanoparticles relative to the total weight of the nanocomposite powder.

In a preferred embodiment, the zirconium dioxide nanoparticles are substantially granular or substantially spherical (e.g. oval or oblong in shape). In certain embodiments, the zirconium dioxide nanoparticles may be of any shape that provides suitable reinforcement to the repaired acrylic denture base. In certain embodiments, the nanoparticles may be in the form of at least one shape selected from the group including, but not limited to, a sphere, a rod, a cylinder, a 3-dimensional form having one or more sides in the form of a rectangle, a triangle, a pentagon, and/or a hexagon, a prism, a disk, a platelet, a fiber, a cube, a cuboid, and tube, and an urchin (e.g. a globular particle possessing a spiky uneven surface).

In a preferred embodiment, the nanoparticles of the present disclosure may be uniform. As used herein, the term “uniform” refers to no more than 10%, preferably no more than 5%, preferably no more than 4%, preferably no more than 3%, preferably no more than 2%, preferably no more than 1% of the distribution of the nanoparticles having a different shape. For example, spherical nanoparticles are uniform and have no more than 1% of nanoparticles in an oblong shape. In certain embodiments, the nanoparticles may be non-uniform. As used herein, the term “non-uniform” refers to more than 10% of the distribution of the nanoparticles having a different shape.

As used herein, granularity refers to the extent to which a material is composed of distinguishable pieces or grains. As used herein, particle size refers to a notion introduced for comparing dimensions of solid particles and may be applied to particles present in granular material and particles that form a granular material (i.e. grain size). As used herein, a particle size refers to the longest linear distance measured from one point on the particle though the center of the particle to a point directly across from it. In a preferred embodiment, the zirconium dioxide nanoparticles of the present disclosure in any of their embodiments have an average granularity and/or average particle size of 50-130 nm, preferably, 55-125 nm, preferably 60-120 nm, preferably 65-115 nm, preferably 70-110 nm, preferably 75-105 nm, preferably 80-100 nm, preferably 82-98 nm, preferably 84-96 nm, preferably 85-95 nm, preferably 86-94 nm, preferably 88-92 nm. As discussed in the Examples sections in more detail, the use of nano-zirconia provides an unexpected result compared to micron sized zirconia (e.g. 1-7 μm, or about 5 μm), where the addition of micron sized zirconia does not significantly increase the transverse strength of unmodified autopolymerized resins, and in some cases actually decreases the transverse strength with higher loadings (e.g. 2 wt % versus 5 wt %), whereas zirconium dioxide nanoparticles (nano-zirconia) significantly improves the transverse strength compared to unmodified autopolymerized resins. Higher loadings (e.g. 3-7 wt %) of nano-zirconia also unexpectedly provides better transverse strength than lower loadings (less than 3 wt %), in contrast to results from micron sized ZrO₂.

As used herein, “dispersity” is a measure of the heterogeneity of sizes of molecules or particles in a mixture. In probability theory and statistics, the coefficient of variation (CV), also known as relative standard deviation (RSD) is a standardized measure of dispersion of a probability distribution. It is expressed as a percentage and may be defined as the ratio of the standard deviation (σ) to the mean (μ, or its absolute value |μ|). The coefficient of variation or relative standard deviation is widely used to express precision and/or repeatability. It may show the extent of variability in relation to the mean of a population. In a preferred embodiment, the nanoparticles of the present disclosure have a narrow size dispersion, i.e. monodispersity. As used herein, “monodisperse”, “monodispersed” and/or “monodispersity” refers to nanoparticles which have a CV or RSD of less than 30%, preferably less than 25%, preferably less than 20%, preferably less than 15%, preferably less than 12%, preferably less than 10%, preferably less than 8%, preferably less than 5%.

In a preferred embodiment, the nanoparticles of the present disclosure are monodisperse with a coefficient of variation or relative standard deviation (ratio of the particle size standard deviation to the particle size mean) of less than 15%, preferably less than 12%, preferably less than 10%, preferably less than 9%, preferably less than 8%, preferably less than 7%, preferably less than 6%, preferably less than 5%, preferably less than 4%, preferably less than 2%. In a preferred embodiment, the nanoparticles are monodisperse and have a particle diameter distribution in a range of 75% of the average particle diameter to 125% of the average particle diameter, preferably 80-120%, preferably 85-115%, preferably 86-114%, preferably 87-113%, preferably 88-112%, preferably 89-111%, preferably 90-110%, preferably 95% of the average particle diameter to 105% of the average nanoparticle diameter.

As used herein, surface area of a material refers to a measure of the total area that the surface of the material occupies. In a preferred embodiment, the zirconium dioxide nanoparticles of the present disclosure in any of their embodiments have an average surface area of 5-15 m²/g, preferably 5.5-14 m²/g, preferably 6-13 m²/g, preferably 6.5-12 m²/g, preferably 7-11 m²/g, preferably 7.5-10.5 m²/g, preferably 8-10 m²/g, preferably 8.5-9.5 m²/g.

The method also involves forming a reinforcement resin by dispersing the nanocomposite powder in a second amount of the monomer liquid comprising methyl methacrylate. The second amount of the monomer liquid may be the same amount, or a different amount, of monomer liquid compared to the amount of monomer liquid employed in painting the repair gap surface. The phrase “second amount of the monomer liquid” therefore denotes that the reinforcement resin is made using separate monomer liquid from that used to paint the repair gap surface, i.e. a first batch of monomer liquid is used for the painting and a second batch of monomer liquid is used to form the reinforcement resin. Preferably the same monomer liquid composition is used (e.g. the same wt % of methyl methacrylate) for both the painting and the dispersing, however this is for convenience and is not required. In a preferred embodiment, the reinforcement resin has a nanocomposite powder to monomer liquid mass ratio in a range of 0.5:1 to 3:1, preferably 1:1 to 2.5:1, preferably 1.5:1 to 2.33:1, preferably 1.75:1 to 2.25:1, preferably 1.8:1 to 2.2:1, preferably 1.9:1 to 2.1:1, or about 2:1.

As those skilled in the art will understand readily, the reinforcement resin may be formed from a blend of several different but compatible materials. Similarly, the reinforcement resin may contain small amounts of specific filler-type additives to enhance polishability, to adjust opacity, to prevent sedimentation, and for other purposes.

In certain embodiments, it is equally envisaged that the nanocomposite powder or reinforcement resin may further comprise one or more inorganic powder or filler in addition to the zirconia nanoparticles. The additional inorganic filler may be a hard, natural or synthetic inorganic oxide selected from a ceramic powder such a silicon oxide (silica), quartz, metal oxides including aluminum, zirconium, titanium and iron oxides, silicates such as, but not limited to, those based on the oxides of lithium, calcium, barium, strontium, magnesium, aluminum, sodium, potassium, cerium, tin, strontium, boron, lead, and mixtures of thereof. Some of the silicates are naturally occurring (e.g., talc, kaolin) and some are not (alumina, zirconium oxide (zirconia)). Other suitable fillers are based on a metal nitride such as silicon nitride or mixtures of metal nitrides. The glasses may or may not have fluoride releasing properties. The filler may comprise more than one type of a particulate material. The powdered particulate preferably comprises discrete and individual particles that are not substantially agglomerated or aggregated. The particles are preferably characterized by having a relatively small particle size and large surface area.

In a preferred embodiment, the method further comprises treating the zirconium dioxide nanoparticles with a silane coupling reagent prior to the dispersing. Good adhesion and dispersion homogeneity of nano-ZrO₂ with the resin matrix effectively improve the properties of the polymer/nanoparticles composite. Therefore, surface modification of nanoparticles with a silane coupling agent, such as 3-(trimethoxysilyl)propyl methacrylate [TMSPM] solution) helps reduce aggregation of nano-ZrO₂ and enhances its compatibility with the polymer, which may result in the improvement of composite properties [Erjun T, Cheng G, Pang X, et al. Synthesis of nano-ZnO₂/PMMA composite micros sphere through emulsion polymerization and UV-Shielding property. J Colloid Polymer Sci. 2005; 284(4):422-428.—incorporated herein by reference in its entirety]. Typical silane derivatives (herein also termed “silanes” or “silane coupling reagent”) suitable for the purpose of the invention include, but are not limited to, silanes bearing a methacrylic functional group such as methacryloxypropyl trimethoxy silane; silanes bearing an epoxy group such as glycidoxy propyl trimethoxy silane or beta-(3,4-epoxycyclohexyl)ethyl trimethoxysilane; silanes comprising amino functional group such as gamaaminopropyl trimethoxy silane, gama-aminopropyl triethoxy silane or N-beta(aminoethyl)gama-aminopropyl trimethoxy silane); silanes comprising a mercapto group such as 3-mercaptopropyl trimethoxy silane; or a mixture thereof, or a mixture of one or more of the above silanes with a nonreactive or an inert silane (i.e., a silane with no functional group to connect to the organic polymer) such as phenyl trimethoxy silane and other phenyl silanes. Addition of an inert silane improves adhesion and adds hydrophobicity. The use of the hydrophobic silanes is beneficial for preventing moisture penetration into the reinforcement resin. The zirconium nanoparticles are preferably pre-treated with the silane derivative or, alternatively, a small amount of a silane derivative may be added to the nanocomposite powder, the monomer liquid, and/or the reinforcement resin. In a preferred embodiment, the silane coupling reagent is 3-(trimethoxysilyl)propyl methacrylate. It is equally envisaged that titanate derivatives may be used in addition to and/or in lieu of the silane coupling reagent.

In a preferred embodiment the treating comprises immersing the zirconium dioxide nanoparticles in a solution comprising 1.0-5.0 g of the silane coupling reagent per liter of the solution, preferably 1.5-4.5 g, preferably 2.0-4.0 g, preferably 2.5-3.5 g, preferably 2.75-3.25 g, or about 3.0 g of the silane coupling reagent per liter of the solution. The solution preferably comprises a solvent. As used herein, the term “solvent” refers to and includes, but is not limited to, water (e.g. tap water, distilled water, deionized water, deionized distilled water), organic solvents, such as ethers (e.g. diethyl ether, tetrahydrofuran, 1,4-dioxane, tetrahydropyran, 1-butyl methyl ether, cyclopentyl methyl ether, di-iso-propyl ether), glycol ethers (e.g. 1,2-dimethoxyethane, diglyme, triglyme), alcohols (e.g. methanol, ethanol, trifluoroethanol, n-propanol, i-propanol, n-butanol, i-butanol, i-butanol, n-pentanol, i-pentanol, 2-methyl-2-butanol, 2-trifluoromethyl-2-propanol, 2,3-dimethyl-2-butanol, 3-pentanol, 3-methyl-3-pentanol, 2-methyl-3-pentanol, 2-methyl-2-pentanol, 2,3-dimethyl-3-pentanol, 3-ethyl-3-pentanol, 2-methyl-2-hexanol, 3-hexanol, cyclopropylmethanol, cyclopropanol, cyclobutanol, cyclopentanol, cyclohexanol), aromatic solvents (e.g. benzene, o-xylene, m-xylene, p-xylene, mixtures of xylenes, toluene, mesitylene, anisole, 1,2-dimethoxybenzene, α,α,α-trifluoromethylbenzene, fluorobenzene), chlorinated solvents (e.g. chlorobenzene, dichloromethane, 1,2-dichloroethane, 1,1-dichloroethane, chloroform), ester solvents (e.g. ethyl acetate, propyl acetate), amide solvents (e.g. dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone), urea solvents, ketones (e.g. acetone, butanone), acetonitrile, propionitrile, butyronitrile, benzonitrile, dimethyl sulfoxide, ethylene carbonate, propylene carbonate, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, and mixtures thereof. As used herein solvent may refer to non-polar solvents (e.g. hexane, benzene, toluene, diethyl ether, chloroform, 1,4-dioxane), polar aprotic solvents (e.g. ethyl acetate, tetrahydrofuran, dichloromethane, acetone, acetonitrile, dimethylformamide, dimethyl sulfoxide) and polar protic solvents (e.g. acetic acid, n-butanol, isopropanol, n-propanol, ethanol, methanol, formic acid, water) and mixtures thereof. In a preferred embodiment, the solvent is acetone.

After treating the zirconium dioxide nanoparticles with the silane coupling agent, a silane coating layer is formed that covers at least a portion of a surface of the zirconium dioxide. In one embodiment, the amount of silane coating layer formed is less than 10%, less than 9%, less than 8%, less than 7%, less than 6%, less than 5%, less than 4%, less than 3%, less than 2%, less than 1% by weight, relative to the combined weight of the silane coating layer and the zirconium dioxide nanoparticles. Further, the silane coating layer is formed with a thickness ranging from about 10 nm, from about 15 nm, from about 20 nm, from about 30 nm, from about 40 nm, from about 50 nm, and up to about 100 nm, up to about 90 nm, up to about 80 nm thick.

In certain embodiment, the reinforcement resin may further comprise a pigment or a mixture of pigments, primarily for aesthetic reasons. Any organic and inorganic pigment is suitable for the purpose of the invention, provided it is not toxic as are some cadmium and lead compounds. The pigment is preferably pre-grinded into one of the components of the system, since it is unlikely to function well if added as a separate powder. The pigment is preferably present in small amounts, e.g., 0.01-1 wt. % relative to the total weight of the reinforcement resin, preferably 0.1-0.8 wt %, preferably 0.2-0.6 wt %, preferably 0.3-0.5 wt % relative to the total weight of the reinforcement resin. In addition to or in lieu of a pigment, the reinforcement resin may further comprise one or more additives. Exemplary suitable additives include, but are not limited to, fluoride-releasing agents; flavorants; fluorescent agents; opalescent agents; ultra-violet stabilizers; anti-oxidants; viscosity modifiers, and the like.

The reinforcement resin may be applied and packed into the repair gap surface using any known techniques such as sprinkling in a “salt and pepper method”, application through a spatula, etc. Preferably the reinforcement resin is applied in excess to the painted repair gap surface of the acrylic denture base to counteract any shrinkage that may occur after self-curing. However, application of an amount less than excess or only partial filling of the repair gap surface may be adequate depending on the type of repair needed and/or the type of prosthesis in need of repair.

In one embodiment, the painting, the applying/packing, and the subsequent auto-polymerization are performed in sequence and the sequence is repeated to form a layered repair gap. For example, the repair gap surface may be painted with an amount of monomer liquid that is less than a total volume of the repair gap, the reinforcement resin may be applied/packed in an amount less than the total volume of the repair gap, and the reinforcement resin may be allowed to autopolymerize (e.g. by waiting for a minute or more) to form a partially filled repair gap. Then additional monomer liquid may be painted onto the partially filled repair gap, followed by another application of reinforcement resin and additional curing etc. to form a layered repair gap. Because each step of the repeated sequence may differ, the end result may be a repair gap having distinct sub-layers of cured reinforcement resin (i.e. sub layer thickness, content, or level of curing) with an identifiable interface between each sub layer.

In certain embodiments, autopolymerization is carried out by a self-curing or autopolymerizing process which starts spontaneously when the monomer liquid and the nanocomposite powder are brought into contact, for example, by mixing. These components of the self-curing or autopolymerizing system are contained in at least two separate containers or compartments of the kit of the invention and mixed immediately prior to application or during application to the dental prosthesis. In one embodiment, the autopolymerizing occurs at a temperature of 20-50° C., preferably 21-40° C., preferably 21-37° C., preferably 22-37° C. For example, the polymerization reaction may be initiated at room temperature (e.g. 21° C.) to the temperature just inside of a subject's mouth (about 37° C.). Therefore, the autopolymerization temperature is different from typical temperatures used to cure resins using heat polymerization processes, such as those used to generate the acrylic denture base. Typical heat polymerization processes involve heating uncured or partially cured resin forming compositions to 60-150° C., preferably 65-125° C., preferably 70-115° C., preferably 74-100° C. Such heat polymerization process are difficult to perform chair side, as discussed previously, and produce dentures having flexural and impact strength properties which differ from those produced by autopolymerizing resins/temperatures.

In one embodiment, the autopolymerizing is performed at a pressure of 10-50 psi, preferably 11-50 psi, preferably 12-40 psi, preferably 13-30 psi, preferably about atmospheric pressure, which is about 14.69 psi.

In a preferred embodiment, the repair gap surface is in a form of at least one selected from the group consisting of a bevel joint, a butt joint, a rabbet surface and a round surface, preferably at least one selected from the group consisting of a bevel joint and a butt joint, most preferably a bevel joint. It is equally envisaged that the repair gap surface may be at least one selected from the group consisting of a square joint, a single-bevel joint, a double-bevel joint, a single-V joint, and double V-joint, a single J-joint, a double J-joint, a single U-joint, and double U-joint, a flange (edge of corner), a flare groove, and the like.

As used herein a beveled edge or bevel joint refers to an edge of a structure that is not perpendicular to the faces of a structure that is not perpendicular to the faces of the piece. In addition, a bevel joint may refer to a v-joint wherein both sides are beveled. In a preferred embodiment the repair gap surface is in a form of a bevel joint. In a preferred embodiment, the repair gap surface is a 20-80° bevel joint, preferably a 25-75° bevel joint, preferably a 30-70° bevel joint, preferably a 35-65° bevel joint, preferably a 40-60° bevel joint, preferably a 42-50° bevel joint, or about a 45° bevel joint. In certain embodiments, the method may further comprise shaping the repair gap surface into a bevel joint, preferably prior to the painting.

For example the butt joint may include shaping the repair gap surface by cutting or carving or otherwise forming a gap in the prosthesis (e.g. the denture) that is 1-6 mm, preferably 2-4 mm, preferably about 2.5 mm wide, while the bevel joint may be formed to produce a gap that is 8-12 mm, preferably 9-11 mm, preferably about 10 mm wide at a first surface and 1-5 mm, preferably 2-4 mm, preferably about 2.5 mm wide at a second surface to produce the beveled angles described above.

As used herein, flexural strength, also known as modulus of rupture, or bend strength, or transverse rupture strength is a material property, defined as the stress in a material just before it yields in a flexure test. The transverse bending test is most frequently employed, in which a specimen having either a circular or rectangular cross section is bent until fracture or yielding using a three point flexural test technique. The flexural strength represents the highest stress experienced within the material at its moment of yield. For example, fracture load can be measured using a three-point bending test on a universal testing machine (e.g. Instron 8871; Instron Co., Norwood, Mass., USA). The specimens may be placed on a three-point flexure apparatus with a 25-75 mm, preferably 50 mm distance between two supports. A 25-75 kgf, preferably 50 kgf load cell is applied at the midpoint of the repaired area with a crosshead speed of 3-7 mm/min, preferably 5 mm/min until the specimen is fractured and the fracture load can then be recorded. In a preferred embodiment, the repaired acrylic denture base formed by the method of the present disclosure in any of its embodiments or the acrylic denture base comprising the acrylic reinforcement resin of the present disclosure in any of its embodiments has a transverse strength of 60-95 MPa, preferably 62-92 MPa, preferably 65-90 MPa, preferably 70-88 MPa, preferably 72-87 MPa, preferably 75-86 MPa, preferably 80-85 MPa. Such transverse strengths are unexpectedly good, and compare favorably or adequately to heat polymerized materials, and are significantly improved compared to unmodified autopolymerized specimens, or autopolymerized specimens modified with well-known modifiers such as glass fibers and micron zirconia.

In one embodiment, the repaired acrylic denture base having a butt joint formed by the method of the present disclosure has a flexural or transverse strength of 80-90 MPa, preferably 81-88 MPa, preferably 81.5-86 MPa, preferably 81.74-85.32 MPa.

In one embodiment, the repaired acrylic denture base having a bevel joint formed by the method of the present disclosure has a flexural or transverse strength of 85-95 MPa, preferably 86-93 MPa, preferably 86.5-92 MPa, preferably 86.91-91.43 MPa.

Although flexural stresses that are counteracted by the flexural strength of the material are a constant phenomenon during mastication, impact strength is also required to prevent accidental dropping or falling of the dentures. As used herein, impact strength is the capability of a material to withstand a suddenly applied load and is express in terms of energy. Because dentures have frenal notches and some scratches on the denture surface that act as common stress concentrated areas and consequently reduce the strength of the dentures, a Charpy-type impact test can be used along with V-shaped notches in the specimens in order to simulate denture borders [Radzi Z, Abu Kasim N H, Yahya N A, et al. Impact strength of an experimental polyurethane-based polymer. Annal Dent Univ Malaya. 2007; 14:46-51.—incorporated herein by reference in its entirety]. Alternatively an Izod impact strength test can be used to measure the impact energy required to fracture a sample. For example, the impact strength test can be performed using a pendulum Charpy-type impact test machine (e.g. Digital Charpy Izod impact tester, XJU 5.5, Jinan Hensgrand Instrument Co., Ltd., Jinan, China). Each specimen is horizontally positioned with a distance of 20-60 mm, preferably 40 mm between the two fixed supports. At room temperature, a drop weight of 0.3-0.7 J, preferably 0.5 J is applied at the mid-span of the specimen on the opposite side to the notch, and the value of the impact strength (kJ/m²) can then be recorded. Volume, modulus of elasticity, distribution of forces, and yield strength affect the impact strength of a material. In order for a material or object to have a high impact strength the stresses must be distributed evenly throughout the object. It also advantageously has a large volume with a low modulus of elasticity and a high material yield strength. In a preferred embodiment, the repaired acrylic denture base formed by the method of the present disclosure in any of its embodiments or the acrylic denture base comprising the acrylic reinforcement resin of the present disclosure in any of its embodiments has an impact strength of 0.96-2.5 kJ/m², 1-2.3 kJ/m², 1.3-2.25 kJ/m², 1.5-2.0 kJ/m², preferably 1.52-1.7 kJ/m², preferably 1.6-2.0 kJ/m², preferably 1.65-1.9 kJ/m², preferably 1.7-1.85 kJ/m², preferably 1.72-1.8 kJ/m².

In one embodiment, the repaired acrylic denture base having a butt joint formed by the method of the present disclosure has an impact strength of 1.27-2.0 kJ/m², 1.27-1.8 kJ/m², 1.3-1.75 kJ/m², 1.37-1.70 kJ/m².

In one embodiment, the repaired acrylic denture base having a bevel joint formed by the method of the present disclosure has an impact strength of 0.95-1.52 kJ/m², 0.95-1.1.4 kJ/m², 0.96-1.3 kJ/m², 0.98-1.1 kJ/m².

According to a second aspect, the present disclosure relates to a kit comprising i) a monomer liquid comprising methyl methacrylate and ii) a nanocomposite powder comprising a) the acrylic polymer powder comprising polymethyl methacrylate and b) the zirconium dioxide nanoparticles disclosed herein (e.g. 1-10 wt % ZrO₂ relative to the nanocomposite powder), wherein the monomer liquid and the nanocomposite powder are suitable for forming a reinforcement resin that autopolymerizes at a temperature of 20-40° C. and a pressure of 10-30 psi.

Preferably, the kit is divided in two or more portions, formulated to be shelf stable, and packaged separately. These separate portions are then mixed in appropriate amounts, for application. The preferred kit includes a first portion which contains the liquid portion (i.e. the monomer liquid), and a second portion which contains the solid portion (i.e. the acrylic polymer powder). The separate components can be packaged in a re-sealable package, cartridge, or capsule, which can be used, closed and stored for multiple applications if desired. Alternatively, the components of the kit can be packaged as one time use sealed packages, cartridges, or capsules, preferably in premeasured amounts, that can be mixed and used without measuring, and where the remaining one time use packaging materials can then be discarded.

Besides the essential ingredients listed above, the kit of the present invention may further comprise at least one ingredient selected from the group consisting of a polymerization accelerator, a cross-linking agent, a co-reactant, an adhesion promoter or coupling reagent such as a silane or titanate derivative, fibers and micro/nanofibers.

According to a third aspect, the present disclosure relates to an acrylic reinforcement resin comprising i) polymethyl methacrylate formed by an autopolymerization reaction of methylmethacrylate at a temperature of 20-40° C., and ii) zirconium dioxide nanoparticles having an average granularity of 50-130 nm and an average surface area of 5-15 m²/g. After curing, the acrylic reinforcement resin provides the repaired acrylic denture base with the flexural or traverse strength properties and the impact strength properties disclosed previously.

According to a fourth aspect, the present disclosure relates to an acrylic denture base, preferably a heat polymerized PMMA acrylic denture base comprising the acrylic reinforcement resin in any of its embodiments having a flexural or transverse strength as disclosed herein.

Having generally described this disclosure, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. The examples below are intended to further illustrate protocols for repairing an acrylic denture base employing an autopolymerizable acrylic reinforcement resin comprising zirconium dioxide nanoparticles and characterizing the repaired acrylic denture base comprising the reinforcement resin of the present disclosure. Further, they are intended to illustrate assessing the properties of these materials and assessing their performance. They are not intended to limit the scope of the claims.

Example 1 General Materials and Methods of Preparing Testing Specimens

In accordance with ANSI/ADA specification number 12, eighty rectangular specimens of heat polymerized acrylic resin with dimensions (65×10×2.5 mm±0.1) were prepared using customized molds [American Dental Association, “Revised American Dental Association Specification no. 12 for denture base polymers,” Journal of the American Dental Association, vol. 90, no. 2, pp. 451-458, 1975.—incorporated herein by reference in its entirety]. Molds were waxed up (Cavex Set Up Wax, Cavex, Netherlands) and then wax patterns were invested in type III dental stone (GC Fujirock EP, Belgium) within a flask (61B Two Flask Compress, Handler Manufacturing USA) and then dewaxed to create the mold space. According to the manufacturer's instructions, heat polymerized acrylic resin (Major Base 20, Major Prodotti Dentari SPA, Italy) was mixed and packed in the dough stage into the mold cavity and a trial closure was done and then the flask was closed and kept under a bench press for 30 minutes. The flask with acrylic resin specimens was processed for 8 hours in a water bath at 74° C. and then the temperature was increased to 100° C. for 1 hour in a thermal curing unit (KaVo Elektrotechnisches Werk GmbH, D-88299, Germany). After curing, the flasks were bench cooled to room temperature prior to deflasking. The excess resin of deflasked specimens was removed with a tungsten carbide bur (HM251 FX 040 HP, Meisinger, USA), the specimens were polished with an acrylic polisher (HM251FX-060, Meisinger, USA), and then stored in distilled water at 37° C. for 48 hours. All specimens were randomly divided into eight groups; one intact and seven repaired groups. Table 1 indicates the tested groups and coding according to the repair material reinforcement.

TABLE 1 Testing groups and coding according to repair material reinforcement Group Code Repair material HC Intact heat polymerized specimens (control) AP Autopolymerized acrylic resin 2GF Autopolymerized acrylic resin reinforced with 2 wt % glass fiber 5GF Autopolymerized acrylic resin reinforced with 5 wt % glass fiber 2ZR Autopolymerized acrylic resin reinforced with 2 wt % zirconia 5ZR Autopolymerized acrylic resin reinforced with 5 wt % zirconia 2NZR Autopolymerized acrylic resin reinforced with 2 wt % nano-zirconia 5NZR Autopolymerized acrylic resin reinforced with 5 wt % nano-zirconia

To create a 3 mm repair gap, repair specimens were placed into a mold and numbered on both ends for reassembling. A mark was drawn at the specimen center and then at a 1.5 mm distance from this mark two lines were drawn on both sides and perpendicular to the long edge of the specimen. These two lines were extended on the surfaces of the mold as a guide for all specimens. At these lines the specimens were cut with a low speed diamond disc (DeguDent, GmbH, REF 59903107, Dentsply, Germany) under profuse irrigation. A standardized 45° bevel joint was prepared by measuring 2.5 mm and drawing a line parallel to the prepared edge. In the same manner, the mold sides were cut at the center measuring 8 mm from the upper surface and 3 mm from the lower surface preserving the mold base intact. Specimens were placed in the mold and cut in bevel dire3ction by a diamond disc guided by lines and mold surfaces to create a repair gap of 3 mm×100 mm×2.5 mm with a 45° bevel joint.

Glass fiber (E-glass; length=3 mm, Shanghai Richem International Co., Ltd., China), zirconia (99.5%, 5 μm, 1314-23-4, Shanghai Richem International Co., Ltd., China), and nano-zirconia powder (99.9%, <100 nm, 1314-23-4, Shanghai Richem International Co., Ltd., China) were weighed using an electronic balance (S-234; Denver Instrument, Germany) in a concentration of 2 wt % and 5 wt % of autopolymerized acrylic resin powder (Major Repair; Major Prodotti Dentari SPA, Italy). Preweighed glass fiber, zirconia, and nano-zirconia powder were separately added to the autopolymerized acrylic resin powder and thoroughly mixed using a mortar and pestle to achieve an equal distribution of particles and uniform color. According to numbering, specimen sections were reassembled into the original mold and fixed creating 3 mm between reassembled sections.

The repair surfaces were treated with the methyl methacrylate monomer for three minutes. Repair was done using the sprinkle-on monomer-polymer method and slightly overfilling the repair gap to compensate for any polymerization shrinkage and finishing procedures. Once the surface of the repair material lost its glaze, the molds and their contents were placed in a pressure chamber containing water (at 40° C.) and at a pressure of 30 lb/inch² (pound-force per square inch) for 15 minutes. After curing, the specimens were removed from the mold, finished, polished, and then put into distilled water and incubated at 37° C. for 48 hours and then tested [R. N. Rached, J. M. Powers, and A. A. Del Bel Cury, “Efficacy of conventional and experimental techniques for denture repair,” Journal of Oral Rehabilitation, vol. 31, no. 11, pp. 1130-1138, 2004.—incorporated herein by reference in its entirety].

Example 2 Evaluation of Transverse Strength of Testing Specimens

To determine transverse strength, fracture load was measured using the three-point bending test on a universal testing machine (INSTRON 8871, Servo Hydraulic system, Merlin 2 software). The specimens were placed on a 3-point flexure apparatus and the support span was 50 mm. Load was applied at the midpoint of the repaired area with crosshead speed of 5 mm/min until the specimen fractured and fracture load was recorded. The transverse strength values of each specimen can be calculated using formula (I).

$\begin{matrix} {{TS} = \frac{3\; {WL}}{2\; {bd}^{2}}} & (I) \end{matrix}$

In this formula, TS is the transverse strength (in MPa), W is the fracture load (N), L is the distance between the two supports, b is the specimen width, and d is the specimen thickness [M. Alkurt, Z. Yesil Duymus, and M. Gundogdu, “Effect of repair resin type and surface treatment on the repair strength of heat-polymerized denture base resin,” Journal of Prosthetic Dentistry, vol. 111, no. 1, pp. 71-78, 2014; and R. Zbigniew and D. Nowakowska, “Mechanical properties of hot curing acrylic resins after reinforced with different kinds of fibers,” International Journal of Biomedical Materials Research, vol. 1, no. 1, pp. 9-13, 2013.—each incorporated herein by reference in its entirety].

Data analysis was performed by using SPSS-20.0, IBM software, Chicago (USA). The results were presented as mean and standard deviations. Repeated measure ANOVA was applied to see the statistical significance of the variables in comparison with the control group and AP. Post hoc least significance (LSD) test was used to see the pairwise comparison of the variables. P value of ≤0.05 was considered a statistically significant result.

Table 2 summarizes the mean value and standard deviation of transverse strength. The statistical analysis revealed that the transverse strength of the HC (control) was the highest strength amongst tested groups (FIG. 1). There were statistically significant differences in transverse strength between the repaired groups 5NZR, 2NZR, 2GF, and 5ZR as compared to AP (P≤0.05). The highest transverse strength values were in the groups 5NZR, 2NZR, and 2GF, respectively. Additionally, 5ZR showed a significant decrease in transverse strength value. There was no significant difference in transverse strength between 2ZR and 5GF with AP.

TABLE 2 Mean, standard deviation (SD) and P values for different concentrations of glass fiber, zirconia, and nano-zirconia reinforcement Group Code Mean ± SD Versus HC Versus AP HC, control 83.01 ± 3.03 — — AP 44.85 ± 3.68 — — 2GF 56.98 ± 2.58** 0.0001 0.001 5GF 42.75 ± 2.45* 0.0001 0.175 2ZR 50.07 ± 2.97* 0.0001 0.064 5ZR 40.21 ± 3.31** 0.0001 0.035 2NZR 65.43 ± 2.62** 0.001 0.0001 5NZR 70.77 ± 2.80** 0.001 0.0001 *Statistical significance of the material with control group only at P ≤ 0.05. **Statistical significance of the material with control as well as AP at P ≤ 0.05.

The in vitro study was carried out to evaluate the reinforcing effect of different concentrations of glass fiber, zirconia, and nano-zirconia on the transverse strength of a repaired denture base. Results revealed that the HC group had the highest transverse strength values amongst all groups. Some reinforced repaired specimens exhibited an increase in transverse strength compared to AP; hence, the null hypothesis that nano-zirconia addition would have no effect was rejected. The transverse strength of AP decreased up to half the value of the HC group. The decrease in transverse strength may be due to the lower strength of autopolymerized acrylic resin; an insufficient polymerization process; and the residual monomer retained at the repair site.

Glass fiber addition to the repair material was found to improve the transverse strength of the repaired denture base and may have advantages related to aesthetics and ease of use. The findings of the current study revealed an increase in the transverse strength of 2GF compared to AP. This increase may be attributed to the fact that glass fiber has a high resilience which allows the stresses to be received by them without permanent deformation. 5GF showed a decrease in transverse strength. This may be explained due to the high fiber content which might affect the bond strength between the repair material and the denture base.

The results of this study showed that the addition of 2ZR improved the transverse strength of the repaired specimens. This increase in transverse strength may be resulting from the transformation of zirconia from the tetragonal to monoclinic phase resulting in absorbing the energy of crack propagation in a process called transformation toughening. Also, in this process, expansion of ZrO₂ crystals occurs and places the crack under a state of compressive stress and thereby arresting the crack propagation. The results showed that the transverse strength decreased in proportion to zirconia concentration. 5ZR additions resulted in a significant decrease in transverse strength compared to AP. This reduction in transverse strength may be caused by many reasons including a higher filler percentage which resulted in more defects that affect the material strength; clustering of the particles within the resin; and the presence of more filler particles after reaching saturation of the matrix leads to interruption in the resin matrix continuity. In contrast, a previous study reported that the transverse strength increased as zirconia content increased.

Results showed that the transverse strength significantly increased after incorporation of 2NZR. This increase in the transverse strength may be due to a good distribution of the nanosize particles which enables them to enter and fill the spaces between polymeric chains resulting in an increased interfacial shear strength between the nanoparticles and polymeric chains which improves the transverse strength. It was also observed that the maximal transverse strength was recorded with 5 NZR, and the increase in nano-zirconia percentage increases the transverse strength, which is in agreement with some previous studies while being in disagreement with other studies.

The clinical implication of the present study is that the incorporation of nano-zirconia into autopolymerized repair resin enhances the strength of a repaired denture base. The study design could not mimic the clinical conditions; hence this limitation affected the testing procedures and mechanical property investigated. Future research to study these materials should focus on simulation of clinical conditions with existing prosthesis and implementation of appropriate tests. According to the results and limitations of this in vitro study, it could be concluded that nano-zirconia may be considered as a new approach for denture base repair. The repairs resulted in significantly higher transverse strength as compared to unreinforced repaired resin.

Example 3 General Materials and Methods of Preparing Testing Specimens

A total of 180 specimens of heat-polymerized acrylic resin (Major.Base.20; Major Prodotti Dentari Spa, Moncalieri, Italy) were prepared. Ninety specimens were prepared for the flexural strength test in dimensions of 65×10×2.5 mm³ in accordance with the ANSI/ADA specification no. 12. The other 90 specimens were prepared for the impact strength test in accordance with the ISO standard 1567:1999/Amd. 1:2003(E) for denture base polymers [ISO 1567. Dentistry—Denture Base Polymer. Geneva: International Organization for Standardization; Geneva, Switzerland. 1999:1-27.—incorporated herein by reference in its entirety]. Specimens for the impact strength test were prepared with dimensions of 50×6×4 mm³, and each specimen was fabricated with a V-shaped notch. The notch depth was ˜0.8 mm across the entire 6 mm width of the specimen, leaving an effective depth of 3.2 mm below the notch.

Specimens for both strength tests were divided into one control group (intact specimens) and two repair groups for each strength test, and these specimens were consequently divided according to the surface design into four butt groups and four bevel groups for each test. Table 3 indicates the specimen grouping and coding according to the specifications. Specimens of repair groups were prepared in two parts (one pair) for each specimen to allow space for the repair material. In order to standardize the butt and 45° bevel joints, modified metal plates were customized and used to prepare the repair groups' specimens. FIG. 2 is a visual summary of molds used for the specimens. Table 4 summarizes the dimensions of molds and plates used for the fabrication of specimens.

TABLE 3 Specimens grouping and coding with specifications according to repair material reinforcement and repair surface Group Code Repair material Control HC Intact heat polymerized specimens Butt joint BTAP Unreinforced autopolymerized acrylic resin 2BTZ 2.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin 5BTZ 5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin 7BTZ 7.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin Bevel joint BVAP Unreinforced autopolymerized acrylic resin 2BVZ 2.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin 5BVZ 5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin 7BVZ 7.5 wt % nano-ZrO₂ reinforced autopolymerized acrylic resin

TABLE 4 Dimensions of molds and plates used for fabrication of specimens Molds Flexural strength molds Impact strength molds Customized 65 × 10 × 2.5 mm³ 50 × 6 × 4 mm³ with V-shaped mold elevation and 0.8 mm height Butt joint Length = 31.25 mm, width = Length = 23.75 mm, width = plate 10.0 ± 0.01 mm, and thickness = 6 ± 0.01 mm, and thickness = 2.50 ± 0.01 mm 4 ± 0.01 mm 45° bevel Length of 31.25 mm for the lower Length of 23.75 mm for the lower joint plate surface and 30 mm for the upper surface and 20 mm for the upper surface with a 45° bevel, width = surface with a 45° bevel, width = 10 ± 0.01 mm, and thickness = 6 ± 0.01 mm, and thickness = 2.5 ± 0.01 mm 4 ± 0.01 mm

For control group specimens, metal molds were waxed up, and then the wax (Cavex Set Up Wax; Cavex, Haarlem, the Netherlands) specimens were invested in type III dental stone (Fujirock EP; GC, Leuvenn, Belgium) within a metal flask (61B Two Flask Compress; Handler Manufacturing, Westfild, N.J., USA). After the flasking procedure was completed, the dental wax was burned out and the mold spaces were cleaned of any wax traces by immersion in hot water and then allowed to dry. After that, a separating media (Isol Major; Major Prodotti Dentari Spa) was applied to the dental stone surfaces and allowed to dry. According to the manufacturer's instructions, heat-polymerized acrylic resin was mixed and packed in the dough stage into the mold cavity; trial closures were then performed, and the flask was closed and kept under a bench press for 30 minutes. Acrylic resin specimens were processed for 8 hours in a water bath at 74° C., and the temperature was then increased to 100° C. for 1 hour in a thermal curing unit (KaVo Elektrotechnisches Werk GmbH, Leutkirch, Germany). After polymerization, flasks were bench cooled at room temperature prior to deflasking. The excess resin of specimens was trimmed with a tungsten carbide bur (HM251FX-040-HP; Meisinger, Centennial, Colo., USA) and polished with acrylic polisher (HM251FX-060; Meisinger, USA). For repair group specimens, each pair of respective plates was painted with petroleum jelly and subjected to the same flasking procedure as the control group. The flask was then opened, and the metal plates were removed, creating mold spaces that were cleaned, packed, processed, and finished as for intact specimens. A digital caliper (EK-1106B; China Hunan E&K Tools Inc., Changsha, China) was used to evaluate pairs of repair group specimens according to the required dimensions (Table 4). Control and repair groups were stored in distilled water at 37° C. for 7 days.

Example 4

General Procedure for the Silanization of Nano-ZrO₂ Particles and Preparation of PMMA/ZrO₂ Nanocomposites

The nano-ZrO₂ powder used (99.9% purity, 1314-23-4; Sigma-Aldrich Co., St Louis, Mo., USA) had an average granularity of 90 nm and surface area of 9±2 m²/g. The addition of the silane coupling agent TMSPM (Shanghai Richem International Co., Ltd. Shanghai, China) to nano-ZrO₂ particles results in the creation of reactive groups on its surface, which allows for adequate adhesion between nanoparticles and the resin matrix. To achieve this 0.3 g of TMSPM was dissolved in 100 mL of acetone to ensure that it would evenly coat the surfaces of the ZrO₂ particles. Thirty grams of ZrO₂ particles were added to the TMSPM/acetone solution and stirred with a magnetic stirrer (Cimarec Digital Stirring Hotplates, SP131320-33Q; Thermo Fischer Scientific, Waltham, Mass., USA) for 60 minutes. Subsequently, a rotary evaporator was used to remove the solvent under vacuum at 60° C. and 150 rpm for 30 minutes. When the sample was dried, it was heated at 120° C. for 2 hours and naturally cooled to obtain the surface-treated nano-ZrO₂.

The silanized nano-ZrO₂ particles were weighed using an electronic balance (S-234; Denver Instrument, Gottingen, Germany) and added in concentrations of 2.5 wt %, 5 wt %, and 7.5 wt % of autopolymerized acrylic polymer powder (Major. Repair, Major Prodotti Dentari Spa). The pre-weighted silanized nano-ZrO₂ was added to the autopolymerized acrylic polymer powder, thoroughly mixed, and stirred for 30 minutes to achieve an equal distribution of particles and to obtain a consistent and uniform color.

Example 5 General Repair Procedures and Statistical Analysis for Testing Repair Specimens

The metal molds that were used to fabricate the control group's specimens were used to hold the reassembled specimens for repair. Repair surfaces were painted with monomer liquid and left for 3 minutes. Next, specimens were place into the mold and fixed, preserving the required repair gap. The digital caliper was used to measure the proper dimensions of the repair area of each specimen and the whole length of each specimen. According to the manufacturer's recommendations, the mixed nanocomposite powder was dispersed in the methyl methacrylate monomer with a powder/liquid mass ratio of 2:1, then the subsequent material was mixed and packed into the repair area, adding an excess amount to compensate for any polymerization shrinkage. Next, the molds holding the repaired specimens were placed into a pressure pot at a temperature of 37° C. and subjected to 30 psi pressure for 30 minutes. After polymerization, the specimens were removed from the molds, and the excess acrylic resin was removed with a tungsten carbide bur at a low speed. Next, all specimens were finished, using a 600-grit abrasive paper under running water, and then stored in distilled water at 37° C. for 48±2 hours. Next, the specimens were subjected to the corresponding tests: flexural strength and impact strength (Table 1).

SPSS software, version 20.0 (IBM Corporation, Armonk, N.Y., USA) was used for statistical data analysis. The results of the flexural strength and impact strength tests were presented as arithmetic mean and standard deviation (SD). For intragroup comparisons, in relation to the control versus various stages of flexural strength and impact strength, a paired sample t-test was used. A P-value ≤0.05 was considered a statistically significant result.

Example 6 Scanning Electron Microscopy (SEM) Analysis of Repair Specimens Scanning Electron Microscope Examination (SEM)

The samples were fixed on metal stubs and immersed in an ultrasonic bath of deionized water for 10 minutes. A sputtering device was used to spatter the specimens with gold (one cycle of 120 seconds) under vacuum. The fractured surfaces were analyzed using a scanning electron microscope (SEM) (Inspect S50, FEI Company; Hillsboro, Oreg., USA), evaluating the evenness of distribution along the interfaces between the acrylic resin matrix and the nano-ZrO₂ particles. The SEM unit operated at 20 kV, WD=15-18 mm, with a spot size range of 25-100 pA. Photomicrographs were made at ×100, ×200, ×400, ×600, ×1,000, ×1,200, ×1,600, and ×2,000 magnifications for visual inspection and three observers noted whether the nature of the failure was cohesive (within the repair material only), adhesive (at the interface of the repair material and the repaired resin), or mixed (within both the interface and the repair materials).

FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8 are representative SEM images showing the fractured surface of the control, bevel, and butt specimens for the flexural strength specimens. FIG. 9, FIG. 10, FIG. 11, FIG. 12, FIG. 13, and FIG. 14 are representative SEM images showing the fractured surfaces of the control, bevel, and butt specimens for the impact strength specimens. FIG. 3 (the control group) shows a smooth surface with small pits displaying features of brittle fracture. FIG. 4 shows fewer irregularities and dimpling. FIG. 5 and FIG. 6 display more irregular surfaces, and the smooth bases (non-uniform lamellar) exhibited ductile fracture features in addition to even distribution of nanoparticles filling all spaces, particularly at high concentrations. FIG. 7 reveals fewer irregularities accompanied by rough surfaces, which increased as the nano-ZrO₂ concentration increased (FIG. 8). FIG. 9 displays good surface characteristics, which changed with increasing nanoparticle concentrations, resulting in loosely bonded clusters and corresponding space formations (FIG. 10, FIG. 11, and FIG. 12). FIG. 13 and FIG. 14 show images at high magnifications, displaying nanoparticle clusters and spaces. The fracture during the flexural strength specimens' analysis revealed that the bevel joint group suffered primarily cohesive failure, while the butt joint group presented primarily adhesive failure (Table 5).

TABLE 5 Fracture mode of flexural strength repair specimens at both fracture ends Sample Butt Bevel Fracture mode BTAP 2BTZ 5BTZ 7BTZ BVAP 2BVZ 5BVZ 7BVZ Cohesive 1 3 1 8 8 7 6 Adhesive 9 9 7 9 2 1 1 2 Mixed 1 1 2 2

Example 7 Evaluation of Flexural Strength of Repair Specimens

To determine flexural strength, fracture load was measured using a three-point bending test on a universal testing machine (Instron 8871; Instron Co., Norwood, Mass., USA). The specimens were placed on a three-point flexure apparatus with a 50 mm distance between two supports. A 50 kgf load cell was applied at the midpoint of the repaired area with a crosshead speed of 5 mm/min until the specimen fractured. Fracture load was recorded. The formula S=3WL/2bd² was used to calculate the flexural strength values of each specimen, wherein S is the flexural strength (MPa), W is the fracture load (N), L is the distance between the two supports, b is the specimen width, and d is the specimen thickness.

Table 6 summarizes the mean and SD of flexural strength for the tested specimens. The statistical analysis showed that the flexural strength of the control group was significantly better than all repaired groups (P<0.05), excepting the bevel group reinforced with 7.5% nano-ZrO₂ (P>0.05). Within each bevel group and butt group, there was a statistically significant difference between the nano-ZrO₂ reinforced autopolymerized acrylic resin group and the nano-ZrO₂ unreinforced autopolymerized group (P<0.05). Between reinforced groups, the bevel group's strength, reinforced with 7.5% nano-ZrO₂ (91.43 MPa), was significantly higher than the other reinforced groups (P<0.05). The lowest flexural strength value was found in the butt group reinforced with 2.5% nano-ZrO2 (81.74 MPa). Furthermore, there was no significant differences between the butt and bevel groups reinforced with 5% nano-ZrO2, while there was a significant difference between the 2.5% group and the 7.5% group (P<0.05; Table 6).

TABLE 6 Mean value and SD of flexural strength of tested specimens (MPa) Sample Butt Bevel Control AP 2.5% 5% 7.5% AP 2.5% 5% 7.5% Mean (SD) 92.43 53.29 81.74 85.32 84.51 54.75 86.91 87.64 91.43 P-value 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) 0.005^(a) 0.003^(a) 0.123^(a) — 0.001^(b) 0.001^(b) 0.001^(b) — 0.001^(b) 0.001^(b) 0.001^(b) — — — — 0.156^(c) 0.020^(c) 0.123^(c) 0.001^(c) ^(a)Shows P-value of significance of mean effect of flexure strength of each repaired group compared to that of the control group ^(b)Shows P-value of significance of mean effect of flexure strength compared to that of the conventional group (AP) within each group (butt/bevel). ^(c)Shows P-value of significance of mean effect of flexure strength compared between butt versus bevel groups.

The choice of repair material depended primarily on the strength of the repair material, the repair surface design, and the choice of repair material reinforcement. Nano-ZrO₂ incorporation into a PMMA denture base resin had a significantly beneficial effect on the material's mechanical properties. The resin/filler interface adhesion was an important factor that affected PMMA/nano-ZrO₂ composite properties. Initially, it was assumed that the modification of nano-ZrO₂ particles with a silane coupling agent improved the bonding between reinforcement materials and the PMMA resin matrix, which consequently increased the PMMA/nano-ZrO₂ composite material's strength. The present in vitro study evaluated the effects of incorporation of nano-ZrO₂ particles into repair resin and the repair surface design on the flexural strength and impact strength of repaired denture base resins.

The results showed that repairs using unreinforced autopolymerized resin revealed a significant decrease in flexural strength for both butt and bevel surface joints in comparison with the control group, with the exception of the bevel group reinforced with 7.5% nano-ZrO₂. Otherwise, all the reinforced repaired groups demonstrated significantly increased flexural strength values in comparison to the unreinforced autopolymerized resin group. Reinforcement of a repaired denture base resin with nano-ZrO₂ resulted in an increase in its flexural strength. In this study, the addition of 2.5% nano-ZrO₂ particles, 5% nano-ZrO₂ particles, and 7.5% nano-ZrO₂ particles showed a statistically significant increase in flexural strength compared to the unreinforced autopolymerized resin. However, with the addition of 7.5% nano-ZrO₂ particles, the maximum flexural strength value was recorded, but it had no statistical significance compared to the control group. This increase in flexural strength could be attributed to nano-ZrO₂ particle sizes, their distribution within the repair material, and the silanization process, along with the joint's surface design. In addition, the transformation of ZrO₂ from the tetragonal to monoclinic phase may have resulted in absorbing the energy of crack propagation in a process called transformation toughening. In addition, during this process, the expansion of ZrO₂ crystal may occur and place the crack under a state of compressive stress, which may lead to the arresting of crack propagation. Based on the SEM analysis shown in FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8, this increase in flexural strength might be due to the good distribution of nano-size particles and interstitial filling of acrylic resin with ZrO₂, which interrupted the crack propagation.

The topology of the fractured surface of each specimen described by SEM was studied for changes according to nano-filler concentrations. Smooth surfaces exhibited brittle fracture characteristics, while more uniformly distributed irregularities exhibited ductile characteristics. SEM analysis in FIG. 4, FIG. 5, and FIG. 6 shows uniformly distributed irregularities with a dimpled appearance, representing ductile fractures.

The amount of filler used to reinforce acrylic resins and the filler-to-resin interactions were important factors affecting mechanical properties. The percentage of nano-ZrO₂ used must be dispersed evenly within the resin matrix without interrupting its continuity. Flexural strength was proportional to the nano-ZrO₂ percentage; the repair resin's flexural strength increased as the nano-ZrO₂ concentration increased. This finding was in agreement with some previous studies, but in disagreement with others. These differences could be explained on the basis of studies having different methodologies or practices, such as dissimilar testing procedures, various nanoparticle concentrations, or different filler surface modifications.

Repair material and the repaired acrylic resin interface with the original material were addition important factors that affected the overall repaired structure's mechanical properties. Moreover, the joint surface design has been proven to have a major impact on the strength of the repaired acrylic resin. Cohesive failure primarily occurred with the bevel joint design, demonstrating that beveling of repair surface design increased the flexural strength. This strength increase might be due to the 45° beveling, which increased the interface surface area and consequently provided a wide bond area. Mechanically, the 45° beveling might also shift the damaged area's tensile stress to the shear stress at the interface of the repaired specimens. In contrast, the butt joint primarily experienced adhesive failure. The lack of adhesion could be attributed to the small surface area provided by the butt joint, which may result in decreased flexural strength.

Example 8 Evaluation of Impact Strength of Repair Specimens

An impact strength test was performed using a pendulum Charpy-type impact test machine (Digital Charpy Izod impact tester, XJU 5.5, Jinan Hensgrand Instrument Co., Ltd., Jinan, China). Each specimen was horizontally positioned with a distance of 40 mm between the two fixed supports. At room temperature, a drop weight of 0.5 J was applied at the mid-span of the specimen on the opposite side to the notch, and the value of the impact strength (kJ/m²) was digitally recorded.

Table 7 shows the mean and SD of impact strength for the tested groups. The mean values of all repaired groups were significantly lower than those of the control group (2.69 kJ/m2; P<0.05). In the repaired groups, the lowest impact strength value was found in the bevel group reinforced with 7.5% nano-ZrO₂ (0.96 kJ/m²), while the highest value was seen in the butt group reinforced with 2.5% non-ZrO₂ (1.70 kJ/m²). In comparison to the unreinforced autopolymerized acrylic resin group, a significant increase in the impact strength was found in the butt group reinforced with 2.5% nano-ZrO₂ particles, while a significant decrease was seen in the bevel group reinforced with 7.5% nano-ZrO₂ particles. Furthermore, no significant differences were found within the butt groups between the unreinforced autopolymerized group and the 5% nano-ZrO₂ or 7.5% nano-ZrO₂ reinforced autopolymerized acrylic resin group. There were no significant differences among the bevel groups between the unreinforced autopolymerized groups and the 2.5% nano-ZrO₂ reinforced resin or the unreinforced autopolymerized groups and the 5% nano-ZrO₂ reinforced autopolymerized acrylic resin group.

TABLE 7 Mean value and SD of impact strength of tested specimens Sample Butt Bevel Control AP 2.5% 5% 7.5% AP 2.5% 5% 7.5% Mean (SD) 2.69 1.26 1.70 1.37 1.27 1.46 1.52 0.98 0.96 P-value 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) 0.001^(a) — 0.001^(b) 0.063^(b) 0.938^(b) — 0.461^(b) 0.652^(b) 0.001^(b) — — — — 0.006^(c) 0.057^(c) 0.584^(c) 0.001^(c) ^(a)Shows P-value of significance of mean effect of impact strength of each repaired group compared to that of the control group ^(b)Shows P-value of significance of mean effect of impact strength compared to that of the conventional group (AP) within each group (butt/bevel). ^(c)Shows P-value of significance of mean effect of impact strength compared between butt versus bevel groups.

The results of the current study showed that all the repaired groups shared a significant decrease in their impact strength compared to the control group. Among the repaired groups, there was a significant increase in the impact strength with the 2.5% nano-ZrO₂ with a bevel-shaped repair, and there was a significant decrease with the 7.5% nano-ZrO₂ with a butt-shaped repair compared to the unreinforced autopolymerized repair groups. This factor implied that the addition of 2.5% nano-ZrO₂ provided the nanocomposite with its maximum impact strength, but the addition of 5% nano-ZrO₂ and 7.5% nano-ZrO₂ reduced the impact strength values. The lowest mean value for impact strength occurred with the 7.5% nano-ZrO₂. This reduction in impact strength may be due to the agglomeration of nano-ZrO₂ at 5 wt % and 7.5 wt %, which resulted in loosely bonded cluster formations, where crack propagation may occur and affect the impact strength. The SEM analysis in FIG. 13 and FIG. 14 showed cluster formations and voids on both sides of fractures, which could explain the decreased impact strength. Statistical analysis showed no significant difference between the reinforced bevel and butt groups, except for 7.5% nano-ZrO₂ with a bevel, which demonstrated significantly reduced impact strength. The existence of a V-notch confirmed that the specimens were broken at the same point during testing. Therefore, the surface design did not significantly affect the repairs' impact strength.

Based on the results the incorporation of nano-ZrO₂ into repair resins may improve the repair strength of the material and increase the fracture resistance of the repaired denture base. This justifies the clinical importance of incorporation of nano-ZrO₂ into repair resins compared to unreinforced autopolymerized repair resins. Reinforcement materials, surface design, and repair technique were major factors that primarily affected the repairs' strength. Therefore, the choice of repair reinforcement material in combination with the repair surface design was of major importance in obtaining the best mechanical properties for repair resins.

In conclusion, the incorporation of nano-ZrO₂ into a dental bridge repair material, combined with a bevel joint repair surface, improved the flexural strength of the repaired resin denture base. The impact strength of nano-ZrO₂ reinforced specimens increased with a low percentage (2.5%) of nano-ZrO₂ and decreased as the content of nano-ZrO₂ increased (5% and 7.5%).

Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. As will be understood by those skilled in the art, the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the disclosure, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public. 

1: A method of repairing an acrylic denture base having a repair gap surface, the method comprising: painting the repair gap surface of the acrylic denture base with a monomer liquid comprising methyl methacrylate thereby forming a painted repair gap surface; dispersing a nanocomposite powder in a second amount of the monomer liquid comprising methyl methacrylate thereby forming a reinforcement resin, the nanocomposite powder comprising; an acrylic polymer powder comprising polymethyl methacrylate; and zirconium dioxide nanoparticles; applying and packing the reinforcement resin in excess to the painted repair gap surface of the acrylic denture base; and autopolymerizing the reinforcement resin thereby forming a repaired acrylic denture base. 2: The method of claim 1, wherein the nanocomposite powder comprises 1-10 wt % zirconium dioxide nanoparticles relative to the total weight of the nanocomposite powder. 3: The method of claim 1, wherein the zirconium dioxide nanoparticles have an average granularity of 50-130 nm. 4: The method of claim 1, wherein the zirconium dioxide nanoparticles have an average surface area of 5-15 m²/g. 5: The method of claim 1, wherein the reinforcement resin has a nanocomposite powder to monomer liquid mass ratio in a range of 0.5:1 to 3:1. 6: The method of claim 1, wherein the autopolymerizing is performed at a temperature of 20-50° C. 7: The method of claim 1, wherein the autopolymerizing is performed at a temperature of 20-30° C. 8: The method of claim 1, wherein the autopolymerizing is performed at a pressure of 10-50 psi. 9: The method of claim 1, further comprising treating the zirconium dioxide nanoparticles with a silane coupling reagent prior to the dispersing. 10: The method of claim 9, wherein the treating comprises immersing the zirconium dioxide nanoparticles in a solution comprising 1.0-5.0 g of the silane coupling reagent per liter of the solution. 11: The method of claim 9, wherein the silane coupling reagent is 3-(trimethoxysilyl)propyl methacrylate. 12: The method of claim 1, wherein the repaired acrylic denture base has a flexural or transverse strength of 60-95 MPa. 13: The method of claim 1, wherein the repaired acrylic denture base has an impact strength of 1.5-2.5 kJ/m². 14: The method of claim 1, wherein the repair gap surface is in a form of at least one selected from the group consisting of a bevel joint, a butt joint, a rabbet surface, and a round surface. 15: The method of claim 14, wherein the repair gap surface is in a form of a bevel joint. 16: A kit, comprising: a monomer liquid comprising methyl methacrylate; and a nanocomposite powder, comprising; an acrylic polymer powder comprising polymethyl methacrylate; and 1-10 wt % zirconium dioxide nanoparticles relative to the total weight of the nanocomposite powder, wherein the zirconium dioxide nanoparticles have an average granularity of 50-130 nm and an average surface area of 5-15 m²/g; wherein the monomer liquid and the nanocomposite powder are suitable for forming a reinforcement resin that autopolymerizes at a temperature of 20-40° C. and a pressure of 10-30 psi. 17: The kit of claim 16, further comprising at least one selected from the group consisting of a silane coupling reagent, a polymerization accelerator, a cross-linking agent, and an adhesion promoter. 18: The kit of claim 17, further comprising a silane coupling reagent which is 3-(trimethoxysilyl)propyl methacrylate. 19: An acrylic reinforcement resin, comprising: polymethyl methacrylate formed by an autopolymerization reaction of methylmethacrylate at a temperature of 20-40° C.; and zirconium dioxide nanoparticles having an average granularity of 50-130 nm and an average surface area of 5-15 m²/g. 20: An acrylic denture base comprising the acrylic reinforcement resin of claim 19 having a flexural or transverse strength of 60-95 MPa and an impact strength of 1.5-2.5 kJ/m². 